Various processes are known for drying moist bulk materials and particularly sewage sludge. It has long been problematic, however, to handle or treat the moisture-laden exhaust gases that result from the drying processes. These exhaust gases, in addition to being hot and moisture-laden, are usually malodorous and laden with environmentally damaging or even toxic substances.
For this reason, the discharge of these exhaust gases directly into the atmosphere without some form of processing is to be avoided.
Published European Patent Document 0,593,887 and corresponding U.S. Pat. No. 5,428,904 (Rutz) disclose a method for thermally drying pasty materials and particularly sewage sludge that is preferably in pelletized form. In the disclosed method, moisture is extracted from the sewage sludge, except for a remaining residual moisture content, by means of a flow of a drying gas at a temperature below the mobilization or activation temperature of sewage constituent substances or additive substances present in the sludge. In a first gas flow circuit, the moisture is extracted from the drying gas by means of a gas demoisturizer, and the resulting dried gas is recirculated into the dryer. In a second gas flow circuit, the moisture that has been absorbed or taken up by the gas demoisturizer is extracted therefrom by a flow of heated regeneration gas. The tempered and moisture-laden regeneration gas is then exhausted, either through a deodorizing apparatus or directly, into the atmosphere.
While the disclosed method and apparatus according to U.S. Pat. No. 5,428,904 have proved suitable for the intended purposes, it has been found that the separate operations and gas flow circuits for drying the process gas and then for drying the gas demoisturizer, as well as for cleaning the regeneration gas, require relatively high efforts with regard to the necessary equipment and control technology. Furthermore, carrying out the method in such a plant requires a considerable operating and maintenance effort and relatively high operating costs for the operator of the plant.
Austrian Patent 393,379 (Curtius) discloses a method for thermally treating, and particularly drying, a flowable water-containing bulk material, and particularly waste water sludge. In this known method, the previously compacted or granulated material is conveyed through a fluidized bed of a closed fluidized bed apparatus, in which the bulk material is heated by means of installed heating surfaces for producing an inert gas, which essentially comprises water vapor or steam. The fluidized bed is fluidized exclusively by means of this developed gas. The exhaust gases in excess of the gas needed for ventilating and fluidizing the bulk material are conveyed out of the contact fluidized bed dryer through a branch pipe. A cyclone separator or a filter is installed in this branch pipe for cleaning the exhaust gases, and a condenser is installed in this branch pipe for heat recovery. Carrying out the hygenization, i.e. cleaning, and filtering of the exhaust gas requires a relatively high effort with regard to the necessary equipment and control technology, and simultaneously necessitates a certain operating and maintenance effort.
German Patent Laying-Open Document 4,016,100 discloses a method and an apparatus for drying sewage sludge, wherein the sludge is provided into and conveyed through a column-like or silo-like container, while tempered drying air is flowed through the sewage sludge from the bottom to the top of the container. After it has flowed through the sewage sludge, the drying air, which has thus been heated and laden with moisture, is removed from the top of the container via an exhaust pipe by creating a negative pressure in this exhaust pipe by means of a blower. A filter is connected in circuit downstream from the blower in order to remove malodorous substances and the sewage decay gases from the exhaust gas before it is blown out into the surrounding environment. Even though this known sewage sludge dryer seems to be relatively simple in construction, its operation and the treatment of the dryer exhaust gases in order to produce an environmentally acceptable exhaust gas output stream requires a relatively high effort with regard to the necessary equipment and control technology, as well as operating and maintenance efforts. Furthermore, introduction of the drying air at the bottom of the silo-like container requires the use of high pressure blowers or air pumps, depending on the depth of the container and density of the sludge, which in turn requires a rather high energy input for pumping the drying air flow.
German Patent Laying-Open Document 4,013,761 discloses a method for drying pasty and/or particulate bulk materials such as sewage sludge, whereby the sewage sludge is piled onto an air permeable conveyor arrangement in the form of an air permeable heaped layer, which is conveyed through a drying apparatus by the conveyor apparatus. In the drying apparatus, a tempered drying gas flow is caused to flow alternately back and forth through the conveyor apparatus and the heaped layer of sewage sludge, in order to extract water from the sewage sludge. The resulting water-laden drying gas flow is sucked out of the drying apparatus and conveyed to a condenser in which the moisture is condensed out of the gas. The resulting condensate, which is predominately water, can be provided into a water treatment or clarifying plant. The known method apparently does not provide means for preventing the malodorous substances and sewage gases contained in the drying gas from entering and polluting the environment. Moreover, it would presumably be necessary to provide means for neutralizing the condensate extracted from the moisture-laden drying gas, in order to avoid further environmental contamination or equipment contamination by the condensate. The provision of such additional measures necessarily involves a further effort and expense with regard to equipment and control technology, as well as an additional operating and maintenance effort.
German Patent Laying-Open Document 3,923,020 discloses a method for the disposal processing of adhesive or sticky sludges, and particularly lacquer or paint residue sludges that contain environmentally harmful inorganic components that are volatile under the effect of heat. In the known method, the wet sticky sludge is applied onto a band of a substrate material, which is then conveyed through a drying furnace or oven, where the volatile components are dried out of the sludge. Thereby, the sludge is dried and adheres firmly onto the substrate material band to form a composite unit therewith. This composite unit may then be disposed of or recycled. On the other hand, the drying air that contains the moisture, solvents, and other volatile components that have been removed from the sticky sludge is introduced into a combustor and burned, in order to incinerate the dangerous or environmentally undesirable components such as the volatile inorganic components. This subsequent combustion step is expensive to carry out.